Project Summary/Abstract Systemic lupus erythematosus (SLE) is a challenging disease with a significant mortality. Unfortunately, existing treatments have limited efficacy and/or serious side-effects, including the consequences of broad suppression of the immune system. A long-term goal of this research is to improve mankind's understanding of the underlying molecular drivers and mechanisms of SLE to enable the development new and better therapies that address this core pathobiology but spare the broader immune system. This grant addresses the novel hypothesis that long interspersed nuclear elements (LINE1) are the core driver of SLE. They are transcriptionally activated in SLE patients, as well as in closely related diseases like Sjgren's syndrome, and encode for two proteins, the RNA-binding p40/ORF1 and the 150-kDa ORF2 reverse transcriptase (RT), which can generate DNA using LINE1 mRNA, or other mRNAs, as a template. The resulting DNA triggers the cGAS - STING pathway to induce interferon (IFN) ? and/or ? (depending on cell type), which, in turn, appear to play a critical role in driving SLE, as demonstrated by phase 2 efficacy of the type I IFN receptor (IFNAR1)-blocking antibody anifrolumab. In contrast, mAbs neutralizing IFN? alone had much lower efficacy, suggesting that INF? is also important. In agreement with this notion, blocking INF? production by plasmacytoid dendritic cells (pDC) with TLR7 and 9 antagonists have so far failed in clinical trials in SLE. Instead, there is recent evidence that the cGAS - STING pathway is activated in SLE patients. Aims of the research plan are: SPECIFIC AIM 1: To verify the LINE1 RT - IRF3 - IFN axis in SLE patients. SPECIFIC AIM 2. To block the LINE1 - IRF3 - IFN pathway with RTi. Experiments will verify that this pathway occurs in SLE patients, identify measures (biomarkers) that can be used in a clinical trial, validate the RT as a drug target for the treatment of human SLE, and establish the rationale for future clinical testing. The therapeutic implication of this work is that inhibition of the LINE1 RT may halt early SLE as well as prevent new flares in established disease. Since the LINE1 RT does not have any known functions in healthy adult physiology, it appears that inhibiting LINE1 RT would be well tolerated. Indeed, two published papers indicate that several clinically used HIV RT inhibitors also inhibit LINE1 RT, including some of the most recent drugs like emtricitabine and tenofovir alafenamide, which have a particularly good safety record.